Abstract
Abstract. A quantitative comparison study for Raman lidar and ceilometer observations, and for model simulations of mass concentration estimates of smoke particles is presented. Layers of biomass burning aerosol particles were observed in the lower troposphere, at 2 to 5 km height on 4 to 6 June 2019, over Kuopio, Finland. These long-range-transported smoke particles originated from a Canadian wildfire event. The most pronounced smoke plume detected on 5 June was intensively investigated. Optical properties were retrieved from the multi-wavelength Raman polarization lidar PollyXT. Particle linear depolarization ratios (PDRs) of this plume were measured to be 0.08±0.02 at 355 nm and 0.05±0.01 at 532 nm, suggesting the presence of partly coated soot particles or particles that have mixed with a small amount of dust or other non-spherical aerosol type. The layer-mean PDR at 355 nm (532 nm) decreased during the day from ∼0.11 (0.06) in the morning to ∼0.05 (0.04) in the evening; this decrease with time could be linked to the particle aging and related changes in the smoke particle shape properties. Lidar ratios were derived as 47±5 sr at 355 nm and 71±5 sr at 532 nm. A complete ceilometer data processing for a Vaisala CL51 ceilometer is presented from a sensor-provided attenuated backscatter coefficient to particle mass concentration (including the water vapor correction for high latitude for the first time). Aerosol backscatter coefficients (BSCs) were measured at four wavelengths (355, 532, 1064 nm from PollyXT and 910 nm from CL51). Two methods, based on a combined lidar and sun-photometer approach, are applied for mass concentration estimations from both PollyXT and the ceilometer CL51 observations. In the first method, no. 1, we used converted BSCs at 532 nm (from measured BSCs) by corresponding measured backscatter-related Ångström exponents, whereas in the second method, no. 2, we used measured BSCs at each wavelength independently. A difference of ∼12 % or ∼36 % was found between PollyXT and CL51 estimated mass concentrations using method no. 1 or no. 2, showing the potential of mass concentration estimates from a ceilometer. Ceilometer estimations have an uncertainty of ∼50 % in the mass retrieval, but the potential of the data lies in the great spatial coverage of these instruments. The mass retrievals were compared with the Modern-Era Retrospective analysis for Research and Applications, version 2 (MERRA-2) meteorological and aerosol reanalysis. The inclusion of dust (as indicated by MERRA-2 data) in the retrieved mass concentration is negligible considering the uncertainties, which also shows that ceilometer observations for mass retrievals can be used even without exact knowledge of the composition of the smoke-dominated aerosol plume in the troposphere.
Highlights
Wildfires release large amounts of aerosols into the atmosphere, contributing significantly to direct radiative forcing (IPCC 2013, https://www.ipcc.ch/site/assets/uploads/2018/ 02/WG1AR5_Chapter08_FINAL.pdf, last access: 31 March 2021) and affecting cloud optical properties by acting as cloud condensation nuclei (Yu, 2000) or ice nuclei (Prenni et al, 2012)
A difference of ∼ 12 % or ∼ 36 % was found between PollyXT and CL51 estimated mass concentrations using method no. 1 or no. 2, showing the potential of mass concentration estimates from a ceilometer
We applied two methods in this study: with method no. 1, measured backscatter coefficients were converted to backscatter coefficients at 532 nm by the corresponding measured backscatterrelated Ångström exponent and were applied to estimate the mass concentrations; with method no. 2, mass concentrations were estimated from measured backscatter coefficients at each wavelength (355, 532, 1064 nm from PollyXT and 910 nm from CL51) independently
Summary
Wildfires release large amounts of aerosols into the atmosphere, contributing significantly to direct radiative forcing (IPCC 2013, https://www.ipcc.ch/site/assets/uploads/2018/ 02/WG1AR5_Chapter08_FINAL.pdf, last access: 31 March 2021) and affecting cloud optical properties by acting as cloud condensation nuclei (Yu, 2000) or ice nuclei (Prenni et al, 2012). Biomass burning is the dominant global source for carbonaceous aerosols, including organic and black carbon (Andreae, 2019), which can be transported over thousands of kilometers in the atmosphere (Andreae, 1991; Fromm and Servranckx, 2003; Mielonen et al, 2012; Portin et al, 2012). These smoke plumes can mix with other aerosols (such as dust) originating from regional and local sources (Osborne et al, 2019; Tesche et al, 2009). Lidar observations showed that biomass burning aerosols are medium- to highly absorbing particles with an almost spherical shape and small particle size, producing medium to high lidar ratios, low depolarization ratios, and high Ångström exponents (Alados-Arboledas et al, 2011; Amiridis et al, 2009; Baars et al, 2012; Müller et al, 2007; Murayama et al, 2004; Nepomuceno Pereira et al, 2014)
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